If preparation is your goal and you'd rather not carry around a giant toolbox everywhere you go, Swiss+Tech's MMCSSS Micro-Max fits pretty much every tool in your pocket for under $10.

No pocket tool will replace a good, solid toolset simply because of its size, but this one comes about as close as you're going to get. The 19-in-1 keychain-ready multitool offers two hex wrenches (1/4", 7/16"), six screwdrivers (#0,1,2 flat, #0,1,2 Phillips), pliers, bottle opener, wire cutter and stripper, wire crimper, hand drill, file, two rulers, and two rule extensions (both with millimeter and inch measurements). If you need a DIY powerhouse in a pocket size, you can get this solid option on the cheap.

Update: Readers are suggesting a few stronger alternatives. They're a bit more, but may last you longer. Wenochs suggests the Leatherman Micra and deanes the Buck X-Tract.